Low-energy beta particle-emitting radionuclides, such as Technetium-99 (Tc-99), can be difficult to measure in the environment in which they are located. Tc-99 is one of the fission products with a relatively high yield in the thermal neutron fission of Uranium-235 (U-235) or Plutonium-239 (Pu-239), and Tc-99 has a long half-life of approximately 2.1×105 years. Tc-99 is found in the environment due to fallout from past atmospheric nuclear weapons tests and reactor fuel processing. In addition, Tc-99 is mobile in the environment, can move into groundwater, and may constitute a health hazard to humans if taken into the body. For these reasons, it may be desirable to be able to detect the presence of Tc-99. Conventional methods for detecting Tc-99 in soil include sampling the soil and running chemical tests on the soil to determine Tc-99 concentration.
Many conventional radiation detectors are designed to detect higher-energy emitting radionuclides, especially those radionuclides that emit gamma rays that travel a great distance due to the high energy of gamma rays. The only emissions that come from Tc-99 are low-energy beta particles, which absorb quickly and do not travel very far in air or soil. As a result, detectors designed to measure radioactive activity remotely, from a significant distance, may not be useful for detection of such low-energy beta particle-emitting radionuclides.
Further, conventional radiation detectors are not designed to discriminate between higher-energy radiation emitter and low-energy beta emitters, which may be either man made or natural (e.g., K-40). Not being able to discriminate between these emitters makes it difficult to know what contribution of the radiation measured is attributable to Tc-99. Part of the difficulty in using conventional radiation detectors to detect such low-energy emitting radionuclides is due to the expected presence of a number of other radionuclides in the environment being examined, which presence may cause background interference. The background interference may be caused by high-energy gamma- or beta-radiation emitters interacting with the detector. Table 1, below, lists the nuclear properties of Tc-99 along with the expected contaminants and concentrations in an environment (e.g., the vadose zone) to be examined.
TABLE 1Beta EndpointGamma rayExpectedRadiation andEnergyEnergiesConcentrationRadionuclideyield (%)(keV)(keV)(pCi/g)Tc-99β-100%292Low yield   1-14,000gammaCs-137β- −94%511661 (6%)1K-40β- −89% plus low1300146110-40yield positron(11%)emitterU-235A 4-4.5 MeV142.8, 1851-3β- - daughter(46%)productsU-238A-415 to 420 MeV1000 from1-3β- - daughterdaughterproductsTh-232A 3.8-4.0 MeVMultiple1-3β- - daughtergammaproductsraysCo-60β- 100% low3181173 and 2-20yield at 15001332
As indicated by Table 1, a number of other contaminant radionuclides are expected to be present in the environment where Tc-99 may likely be present. These other radionuclides may include Cesium-137 (Cs-137), Potassium-40 (K-40), Uranium-235 (U-235), Uranium-238 (U-238), Thorium-232 (Th-232), and Cobalt-60 (Co-60). Other expected contaminant radionuclides not listed in Table 1, but discussed later, include Strontium-90 (Sr-90), Yttrium-90 (Y-90), and Tritium (H-3).
Referring again to Table 1, Tc-99 exhibits an essentially 100% yield of beta-particle emission with little to no gamma ray emission. The beta endpoint energy is the maximum energy beta particles when the beta particles exit the nucleus before they scatter and lose some energy in the environment. As shown on Table 1, Tc-99 has the lowest beta particle endpoint energy of the radionuclides expected to be present in the target environment. The other radionuclides listed have various yields and energies of beta particle and gamma ray emissions. With the low-energy beta particle emission, the quick absorption of beta particles, and the existence of other higher-energy emitting radionuclides (both beta particles and gamma rays) in the environment causing background interference, detection of low-energy beta particle-emitting radionuclides, such as Tc-99, may prove difficult. This is especially true as Tc-99 is only a beta particle emitter, and no gamma rays, with their accompanying deeper penetration, are emitted.
Some conventional phoswich detectors have been used to discriminate between different types of radiation to determine which types of radiation are present (e.g., discriminate between alpha, beta, and gamma radiation); however, such detectors are not configured to discriminate between different sources of the same radiation type (e.g., between different beta emitters). The inventors have appreciated that there is a need to discriminate between sources of the same radiation type, including low-energy beta emitters such as Tc-99, and to do so in the environment where the radiation source exists without the need to take a material sample to run chemical analyses off-site in a laboratory.